Apparatus for Sizing a Facet Joint

ABSTRACT

Systems and method in accordance with the embodiments of the present invention can include an implant for positioning within a cervical facet joint for distracting the cervical spine, thereby increasing the area of the canals and openings through which the spinal cord and nerves must pass, and decreasing pressure on the spinal cord and/or nerve roots. The implant can be inserted laterally or posteriorly.

CLAIM OF PRIORITY

This application claims priority to U.S. Provisional Application No. 60/679,377, entitled INTER-CERVICAL FACET IMPLANTATION TOOL, filed May 10, 2005 [Our Reference No.: KLYC-01118US9], which application is incorporated herein by reference.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to the following applications: U.S. patent application Ser. No. 11/093,557, entitled INTER-CERVICAL FACET IMPLANT WITH LOCKING SCREW AND METHOD filed Mar. 30, 3005 [Our Reference No.: KLYC-118US5], which claims priority to U.S. Provisional Application No. 60/635,453, entitled, INTER-CERVICAL FACET IMPLANT AND METHOD filed Dec. 13, 2004 [Our Reference No. KLYC-01118US0]; U.S. application Ser. No. 11/053,399, entitled INTER-CERVICAL FACET IMPLANT AND METHOD, filed Feb. 8, 2005 [Our Reference No. KLYC-01118US1]; U.S. application Ser. No. 11/053,624, entitled INTER-CERVICAL FACET IMPLANT AND METHOD, filed Feb. 8, 2005 [Our Reference No. KLYC-01118US2]; U.S. application Ser. No. 11/053,735, entitled INTER-CERVICAL FACET IMPLANT AND METHOD, filed Feb. 8, 2005 [Our Reference No. KLYC-01118US3]; U.S. application Ser. No. 11/053,346, entitled INTER-CERVICAL FACET IMPLANT AND METHOD, filed Feb. 8, 2005 [Our Reference No. KLYC-01122US0]; and U.S. application Ser. No. 11/093,689, entitled INTER-CERVICAL FACET IMPLANT AND METHOD FOR PRESERVING THE TISSUES SURROUNDING THE FACET JOINT, filed Mar. 30, 2005 [Our Reference No. KLYC-01124US0] each of which is incorporated herein in full, by reference.

TECHNICAL FIELD

This invention relates to interspinous process implants.

BACKGROUND OF THE INVENTION

The spinal column is a bio-mechanical structure composed primarily of ligaments, muscles, vertebrae and intervertebral disks. The bio-mechanical functions of the spine include: (1) support of the body, which involves the transfer of the weight and the bending movements of the head, trunk and arms to the pelvis and legs, (2) complex physiological motion between these parts, and (3) protection of the spinal cord and the nerve roots.

As the present society ages, it is anticipated that there will be an increase in adverse spinal conditions which are characteristic of older people. By way of example only, with aging comes an increase in spinal stenosis (including, but not limited to, central canal and lateral stenosis), and facet arthropathy. Spinal stenosis results in a reduction foraminal area (i.e., the available space for the passage of nerves and blood vessels) which compresses the cervical nerve roots and causes radicular pain. Humpreys, S. C. et al., Flexion and traction effect on C5-C6 foraminal space, Arch. Phys. Med. Rehabil., vol. 79 at 1105 (September 1998). Another symptom of spinal stenosis is myelopathy, which results in neck pain and muscle weakness. Id. Extension and ipsilateral rotation of the neck further reduces the foraminal area and contributes to pain, nerve root compression, and neural injury. Id.; Yoo, J. U. et al., Effect of cervical spine motion on the neuroforaminal dimensions of human cervical spine, Spine, vol. 17 at 1131 (Nov. 10, 1992). In contrast, neck flexion increases the foraminal area. Humpreys, S. C. et al., supra, at 1105.

In particular, cervical radiculopathy secondary to disc herniation and cervical spondylotic foraminal stenosis typically affects patients in their fourth and fifth decade, and has an annual incidence rate of 83.2 per 100,000 people (based on 1994 information). Cervical radiculopathy is typically treated surgically with either an anterior cervical discectomy and fusion (“ACDF”) or posterior laminoforaminotomy (“PLD”), with or without facetectomy. ACDF is the most commonly performed surgical procedure for cervical radiculopathy, as it has been shown to increase significantly the foramina dimensions when compared to a PLF.

It is desirable to eliminate the need for major surgery for all individuals, and in particular, for the elderly. Accordingly, a need exists to develop spine implants that alleviate pain caused by spinal stenosis and other such conditions caused by damage to, or degeneration of, the cervical spine.

The present invention addresses this need with implants and methods for implanting an apparatus into at least one facet joint of the cervical spine to distract the cervical spine while preferably preserving mobility and normal lordotic curvature.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a lateral view of two adjacent cervical vertebrae and spinous processes, highlighting the cervical facet joint.

FIG. 2 depicts a lateral view of the cervical spine with spinal stenosis.

FIG. 3A depicts correction of cervical stenosis or other ailment with a wedge-shaped embodiment of the implant of the invention positioned in the cervical facet joint.

FIG. 3B depicts correction of cervical kyphosis or loss of lordosis with a wedge-shaped embodiment of the invention with the wedge positioned in the opposite direction as that depicted in FIG. 3A.

FIG. 4 shows correction of cervical stenosis or other ailment with a further embodiment of the implant of the invention including a screw fixation device for attaching to a single vertebra.

FIG. 5 shows correction of cervical stenosis or other ailment with a further embodiment of the implant of the invention, comprising screw fixation of two implants, one implant fixed to each of two adjacent vertebrae.

FIG. 6 shows cervical spine kyphosis, or loss of lordosis.

FIG. 7 shows correction of cervical kyphosis, or loss of lordosis, with a further embodiment of the implant of the invention comprising two facet implants with screw fixation.

FIG. 8 shows correction of cervical stenosis or other ailment with a further embodiment of the implant of the invention, comprising a facet implant and a keel.

FIG. 9 shows correction of cervical stenosis or other ailment with a further embodiment of the implant of the invention, comprising facet implant, a keel, and screw fixation.

FIG. 10 shows correction of cervical stenosis or other ailment with a further embodiment of the implant of the invention, comprising a facet implant with teeth.

FIG. 11 depicts correction of cervical stenosis or other ailment with a further embodiment of the implant of the invention, comprising a facet implant with teeth and screw fixation.

FIG. 12 depicts correction of cervical stenosis or other ailment with a further embodiment of the implant of the invention, comprising two facet implants having bony ingrowth surfaces.

FIG. 13 depicts correction of cervical stenosis or other ailment with a further embodiment of the implant of the invention, comprising two facet implants having bony ingrowth surfaces and posterior alignment guide.

FIG. 14 shows correction of cervical stenosis or other ailment with a further embodiment of the implant of the invention, comprising two facet implants with increased facet joint contact surfaces.

FIG. 15 shows correction of cervical stenosis or other ailment with a further embodiment of the implant of the invention, comprising two facet implants having bony ingrowth surfaces and screw fixation.

FIG. 16 shows correction of cervical stenosis or other ailment with a further embodiment of the implant of the invention, comprising two facet implants with articular inner surfaces.

FIG. 17 shows correction of cervical stenosis or other ailment with a further embodiment of the implant of the invention, comprising a facet joint implant with a roller.

FIG. 18 shows correction of cervical stenosis or other ailment with a further embodiment of the implant of the invention, comprising a facet joint implant with a plurality of rollers.

FIG. 19 shows correction of cervical stenosis or other ailment with a further embodiment of the implant of the invention, comprising two facet joint implants, screw fixation, and elastic restraint.

FIG. 20 shows correction of cervical stenosis or other ailment with a further embodiment of the implant of the invention, comprising two facet joint implants, screw fixation, and spring restraint.

FIG. 21 shows correction of cervical stenosis or other ailment with a further embodiment of the implant of the invention, comprising two facet joint implants, screw fixation, and magnetic restraint.

FIG. 22A shows a perspective view of a further embodiment of implant of the invention.

FIG. 22B shows a perspective exploded view of the embodiment of the invention shown in FIG. 22A.

FIG. 23A depicts a posterior view of the embodiment of the implant of the invention shown in FIG. 22A.

FIG. 23B shows a posterior view of a locking plate of the embodiment of the implant of the invention shown in FIG. 22A.

FIG. 24A depicts a lateral side view of the embodiment of the implant of the invention shown in FIG. 22A.

FIG. 24B shows a lateral side view of the keel of the locking plate of the embodiment of the implant of the invention shown in FIG. 22A.

FIG. 25A shows a perspective view of a further embodiment of the implant of the invention.

FIG. 25B shows a side view of the embodiment of the implant of the invention in FIG. 25A, having a curved, uniformly-thick artificial facet joint spacer or inter-facet spacer including a tapered end

FIG. 26A shows a perspective view of a further embodiment of the implant of the invention.

FIG. 26B shows a posterior perspective view of the embodiment of the implant of the invention depicted in FIG. 26A.

FIG. 27A depicts a side view of the embodiment of the implant of the invention shown in FIGS. 26A and 26B.

FIG. 27B shows a posterior view of the embodiment of the implant of the invention shown in FIGS. 26A, 26B, and 27A, implanted in the cervical spine.

FIG. 28A depicts a posterior perspective view of a further embodiment of the implant of the invention.

FIG. 28B depicts a side view of the embodiment of the implant of the invention shown in FIG. 28A.

FIG. 29A depicts a side view of an embodiment of a sizing tool of the invention.

FIG. 29B depicts a top view of an embodiment of the sizing tool of the invention depicted in FIG. 29A.

FIG. 29C depicts a perspective view of an embodiment of the sizing tool of the invention depicted in FIGS. 29A-B.

FIG. 29D depicts a side view of the head of the sizing tool of the invention depicted in FIG. 29A

FIG. 29E depicts a cross-sectional view of the head of the sizing tool of the invention depicted in FIGS. 29A-C.

FIG. 30 is a flow diagram of an embodiment of a method of the invention.

FIG. 31A is posterior view of a further embodiment of the implant of the invention.

FIG. 31B is a side view of an embodiment of a locking screw of the implant of the invention depicted in FIG. 31A.

FIG. 32 is a posterior view of a further embodiment of the implant of the invention.

FIGS. 33A and 33B depict initial and final insertion positions of the embodiment of the invention depicted in FIG. 32.

FIG. 34A is a posterior view of a further embodiment of the implant of the invention.

FIG. 34B is a side view of a further embodiment of the implant of the invention.

FIG. 35A is a perspective view of an embodiment of the implantation tool of the invention.

FIG. 35B is a perspective view of the engagement head of the implantation tool of the invention.

DETAILED DESCRIPTION

Embodiments of the present invention provide for a minimally invasive surgical implantation method and apparatus for cervical spine implants that preserves the physiology of the spine. In particular, embodiments provide for distracting the cervical spine to increase the foraminal dimension in extension and neutral positions. Such implants, when implanted in the cervical facet joints, distract, or increase the space between, the vertebrae to increase the foraminal area or dimension, and reduce pressure on the nerves and blood vessels of the cervical spine.

The facet joints in the spine are formed between two vertebrae as follows. Each vertebra has four posterior articulating surfaces: two superior facets and two inferior facets, with a superior facet from a lower vertebra and an inferior facet of an upper vertebra forming a facet joint on each lateral side of the spine. In the cervical spine, the upward inclination of the superior articular surfaces of the facet joints allows for considerable flexion and extension, as well as for lateral mobility. Each facet joint is covered by a dense, elastic articular capsule, which is attached just beyond the margins of the articular facets. The capsule is larger and looser in the cervical spine than in the thoracic and lumbar spine. The inside of the capsule is lined by a synovial membrane which secretes synovial fluid for lubricating the facet joint. The exterior of the joint capsule is surrounded by a capsular ligament. It is this ligament and the joint capsule that must be cut in the embodiments of the method described herein for inserting the artificial facet joint.

In a specific preferred embodiment, an implanted interfacet spacer of 1.5 mm to 2.5 mm in width can result in interfacet distraction that increases foraminal dimension in extension and neutral. Other interfacet spacer dimensions also are contemplated by the invention described herein below. The present embodiments also preserve mobility of the facet joints.

Further embodiments of the present invention accommodate the distinct anatomical structures of the spine, minimize further trauma to the spine, and obviate the need for invasive methods of surgical implantation. Embodiments of the present invention also address spinal conditions that are exacerbated by spinal extension.

FIG. 1 shows a simplified diagram of a portion of the cervical spine, focusing on a cervical facet joint 1 formed between two adjacent cervical vertebrae. The spinous processes 3 are located posteriorly and the vertebral bodies 5 are located anteriorly, and a nerve root canal 7 is visible. Each vertebra has four posterior articulating surfaces: two superior facets and two inferior facets, with a superior facet from a lower vertebra and an inferior facet of an upper vertebra forming a facet joint on each lateral side of the spine. In the cervical spine, the upward inclination of the superior articular surfaces of the facet joints allows for considerable flexion and extension, as well as for lateral mobility. Each facet joint is covered by a dense, elastic articular capsule, which is attached just beyond the margins of the articular facets. The capsule is large and looser in the cervical spine than in the thoracic and lumbar spine. The inside of the capsule is lined by a synovial membrane which secretes synovial fluid for lubricating the facet joint. The exterior of the joint capsule is surrounded by a capsular ligament. It is this ligament that may be pushed out of the way in the embodiments of the method for inserting the artificial facet joint, described herein.

FIG. 2 depicts cervical foraminal stenosis. From the drawing, the nerve root canal 7 is narrowed relative to the nerve root canal 7 depicted in FIG. 1. The spinal canal and/or intervertebral foramina also can be narrowed by stenosis. The narrowing can cause compression of the spinal cord and nerve roots.

FIG. 3A shows a first embodiment 100 of the present invention, which is meant to distract at least one facet joint, in order to increase the dimension of the neural foramen while retaining facet joint mobility. The wedge-shaped embodiment or inter-facet spacer 100 is a wedge-shaped implant that can be positioned in the cervical facet joint 101 to distract the joint and reverse narrowing of the nerve root canal 107. In this embodiment or inter-facet spacer 100, the implant is positioned with the narrow portion of the wedge facing anteriorly. However, it is also within the scope of the present invention to position embodiment or inter-facet spacer 100 (FIG. 3B) with the wide portion of the wedge facing anteriorly, to correct for cervical kyphosis or loss of cervical lordosis.

It is to be understood that implants in accordance with the present invention, and/or portions thereof can be fabricated from somewhat flexible and/or deflectable material. In these embodiments, the implant and/or portions thereof can be made out of a polymer, such as a thermoplastic. For example, in one embodiment, the implant can be made from polyketone, known as polyetheretherketone (“PEEK”). Still more specifically, the implant can be made from PEEK 450G, which is an unfilled PEEK approved for medical implantation available from Victrex of Lancashire, Great Britain. Other sources of this material include Gharda located in Panoli, India. PEEK has the following approximate properties:

Property Value Density 1.3 g/cc Rockwell M 99 Rockwell R 126 Tensile Strength 97 MPa Modulus of Elasticity 3.5 GPa Flexural Modulus 4.1 GPa

The material specified has appropriate physical and mechanical properties and is suitable for carrying and spreading a physical load between the adjacent spinous processes. The implant and/or portions thereof can be formed by extrusion, injection, compression molding and/or machining techniques.

In some embodiments, the implant can comprise, at least in part, titanium or stainless steel, or other suitable implant material which is radiopaque, and at least in part a radiolucent material that does not show up under x-ray or other type of imaging. The physician can have a less obstructed view of the spine under imaging, than with an implant comprising radiopaque materials entirely. However, the implant need not comprise any radiolucent materials.

It should be noted that the material selected also can be filled. For example, other grades of PEEK are also available and contemplated, such as 30% glass-filled or 30% carbon-filled, provided such materials are cleared for use in implantable devices by the FDA, or other regulatory body. Glass-filled PEEK reduces the expansion rate and increases the flexural modulus of PEEK relative to that unfilled PEEK. The resulting product is known to be ideal for improved strength, stiffness, or stability. Carbon-filled PEEK is known to enhance the compressive strength and stiffness of PEEK and to decrease its expansion rate. Carbon-filled PEEK offers wear resistance and load-carrying capability.

In this embodiment or inter-facet spacer 100, the implant is manufactured from PEEK, available from Victrex. As will be appreciated, other suitable similarly biocompatible thermoplastic or thermoplastic polycondensate materials that resist fatigue, have good memory, are flexible, and/or deflectable, have very low moisture absorption, and good wear and/or abrasion resistance, can be used without departing from the scope of the invention. The spacer also can be comprised of polyetherketoneketone (“PEKK”). Other material that can be used include polyetherketone (“PEK”), polyetherketoneetherketoneketone (“PEKEKK”), and polyetheretherketoneketone (“PEEKK”), and generally a polyaryletheretherketone. Further, other polyketones can be used as well as other thermoplastics. Reference to appropriate polymers that can be used in the implant can be made to the following documents, all of which are incorporated herein by reference. These documents include: PCT Publication WO 02/02158 A1, dated Jan. 10, 2002, entitled “Bio-Compatible Polymeric Materials”; PCT Publication WO 02/00275 A1, dated Jan. 3, 2002, entitled “Bio-Compatible Polymeric Materials; and, PCT Publication WO 02/00270 A1, dated Jan. 3, 2002, entitled “Bio-Compatible Polymeric Materials.” Other materials such as Bionate®, polycarbonate urethane, available from the Polymer Technology Group, Berkeley, Calif., may also be appropriate because of the good oxidative stability, biocompatibility, mechanical strength and abrasion resistance. Other thermoplastic materials and other high molecular weight polymers can be used.

Turning now to FIG. 4, the embodiment 200 of the implant has a joint insert or inter-facet spacer 210, also herein referred to as an artificial facet joint or inter-facet spacer, that is positioned in the cervical facet joint 101. The joint insert or inter-facet spacer 210 can be wedge-shaped with the narrow part of the wedge facing anteriorly. Alternatively, the joint insert or inter-facet spacer 210 need not be wedge-shaped but can be of substantially uniform thickness, the thickness determined by an individual patient's need for distraction of the cervical facet joint 201. As with embodiment 100, one objective of this embodiment is facet joint distraction, and joint mobility after implantation. The joint insert 210 is continuous with a posterior sheath 220 bent at an angle from the joint insert or inter-facet spacer 210 to align substantially parallel with the bone. The posterior sheath can lie against the lamina, preferably against the lateral mass. The posterior sheath 220 can have a bore 230 which can accept a bone screw 240. Alternatively, the bore 230 can accept any other appropriate and/or equivalent fixation device capable of fixing the embodiment 200 to the spine. The device is thereby affixed to the vertebra, preferably by fixing to the lateral mass.

FIG. 5 shows embodiment 300, which is the use of two embodiments 200, each fixed to one of two adjacent cervical vertebrae. As with embodiment 200, the implanted facet joint is distracted and joint mobility is retained. A joint insert or inter-facet spacer 310 from each of the two implants is inserted and positioned in the cervical facet joint 301. In this embodiment, the joint inserts 310 are substantially flat and parallel to each other and are not wedge-shaped. Alternatively, the joint inserts or inter-facet spacers 310 can together define a wedge-shaped insert that is appropriate for the patient. The two joint inserts or inter-facet spacers 310 combined can have, by way of example, the shape of the joint insert or inter-facet spacers 210 in FIG. 4. Embodiment 300 then can be fixed to the spine with a screw 340 or any other appropriate fixation device, inserted through a bore 330 in the posterior sheath 320. The posterior sheath 320 can be threaded to accept a screw. The screw can be embedded in the lamina, preferably in the lateral mass, where possible.

It is within the scope of the present invention to use and/or modify the implants of the invention to correct cervical spine kyphosis, or loss of lordosis. FIG. 6 depicts a cervical spine lordosis. FIG. 7 demonstrates an embodiment 400 which contemplates positioning two implants to correct for this spinal abnormality while retaining facet joint mobility. The joint insert or inter-facet spacer 410 of each implant is shaped so that it is thicker at its anterior portion. Alternatively, the implants can be shaped to be thicker at the posterior ends, for example as depicted in FIG. 3A. The posterior sheath 420 of each implant is bent at an angle from the joint insert or inter-facet spacer 410 to be positioned adjacent to the lateral mass and/or lamina, and has a bore 430 to accept a screw 440 or other appropriate and/or equivalent fixation means to fix the embodiment 400 to the spine, preferably to the lateral mass. The placement of two joint inserts or inter-facet spacers 410 in the cervical facet joint 401 distracts the facet joint, which shifts and maintains the vertebrae into a more anatomical position to preserve the physiology of the spine.

FIG. 8 shows a further embodiment 500 of the implant of the invention, wherein the joint insert or inter-facet spacer 510 has a keel 550 on an underside of the joint insert or inter-facet spacer 510. The keel 550 can be made of the same material or materials set forth above. The surfaces of the keel 550 can be roughened in order to promote bone ingrowth to stabilize and fix the implant 500. In other embodiments, the keel 550 can be coated with materials that promote bone growth such as, for example, bone morphogenic protein (“BMP”), or structural materials such as hyaluronic acid “HA,” or other substances which promote growth of bone relative to and into the keel 550.

The keel 550 can be embedded in the facet bone, to facilitate implant retention. The keel 550 can be placed into a channel in the facet bone. The channel can be pre-cut. Teeth (not shown), preferably positioned posteriorly, also may be formed on the keel 550 for facilitating retention of the implant 500 in the cervical facet joint 501. As noted above, the joint insert or inter-facet spacer 510 can be substantially flat or wedge-shaped, depending upon the type of distraction needed, i.e., whether distraction is also necessary to correct abnormal curvature or lack of curvature in the cervical spine. Because the joint is not fused, mobility is retained, as with the embodiments described above and herein below.

FIG. 9 illustrates that a further embodiment 600 of the implant of the invention can have both screw fixation and a keel 650 for stability and retention of the implant 600. On embodiment 600, the joint insert or inter-facet spacer 610 is continuous with a posterior sheath 620 having a bore hole 630 to accept a screw 640 which passes through the bore 630 and into the bone of the vertebrae, preferably into the lateral mass, or the lamina. The bore 630 can be threaded or not threaded where it is to accept a threaded screw or equivalent device. Alternatively, the bore 630 need not be threaded to accept a non-threaded equivalent device. The keel 650 is connected with the joint insert or inter-facet spacer 610 and embeds in the bone of the cervical facet joint 601 to promote implant retention.

A further alternative embodiment 700 is illustrated in FIG. 10. In this embodiment 700, the joint insert 710 has on a lower side at least one tooth 760. It should be clear to one of ordinary skill in the art that a plurality of teeth 760 is preferable. The teeth 760 are able to embed in the bone of the cervical facet joint 701 to facilitate retention of the implant 700 in the joint 701. The teeth 760 can face in a direction substantially opposite the direction of insertion, for retention of the implant 700. As above, the joint insert or inter-facet spacer 710 can be wedge-shaped or substantially even in thickness, depending upon the desired distraction. Because the implant distracts and is retained without fusion, facet joint mobility is retained.

FIG. 11 depicts a further embodiment 800 of the implant of the invention. In this embodiment 800, the joint insert or inter-facet spacer 810 is continuous with a posterior sheath 820 having a bore 830 for accepting a fixation device 840, as described above. The fixation device 840 can be a screw which fits into a threaded bore 830; alternatively, the fixation device 830 can be any other compatible and appropriate device. This embodiment 800 further combines at least one tooth 860 on an underside of the joint insert or inter-facet spacer 810 with the posterior sheath 820, bore 830 and fixation device 840 to address fixation of the implant 800 in a cervical facet joint 801. It will be recognized by one of ordinary skill in the art that the implant 800 can have a plurality of teeth 860 on the underside of the joint insert or inter-facet spacer 810.

FIG. 12 shows yet another embodiment 900 of an implant of the present invention. In this embodiment 900, the joint inserts or inter-facet spacers 910 of two implants 900 are positioned in a cervical facet joint 901. As described above, the joint inserts or inter-facet spacers 910 can be wedge-shaped as needed to restore anatomical curvature of the cervical spine and to distract, or the joint inserts or inter-facet spacers 910 can be of substantially uniform thickness. The implants 900 each comprise a joint insert or inter-facet spacer 910 with an outer surface 970 that interacts with the bone of the cervical facet joint 901. On the upper implant 900, the surface 970 that interacts with the bone is the upper surface 970 and on the lower implant 900, the surface 970 that interacts with the bone is the lower surface 970. Each surface 970 can comprise a bone ingrowth surface 980 to create a porous surface and thereby promote bone ingrowth and fixation. One such treatment can be with plasma spray titanium, and another, with a coating of sintered beads. Alternatively, the implant 900 can have casted porous surfaces 970, where the porous surface is integral to the implant 900. As a further alternative, the surfaces 970 can be roughened in order to promote bone ingrowth into these defined surfaces of the implants 900. In other embodiments, the surfaces 970 can be coated with materials that promote bone growth such as for example bone morphogenic protein (“BMP”), or structural materials such as hyaluronic acid (“HA”), or other substances which promote growth of bone on other external surfaces 970 of the implant 900. These measures facilitate fixation of the implants 900 in the facet joint, but do not result in fusion of the joint, thereby retaining facet joint mobility, while also accomplishing distraction of the joint.

FIG. 13 depicts yet another embodiment 1000 of the implant of the present invention. In this embodiment 1000, the joint inserts or inter-facet spacers 1010 of two implants 1000 are positioned in a cervical facet joint 1001. As described above, the joint inserts or inter-facet spacers 1010 can be wedge-shaped as needed to restore anatomical curvature of the cervical spine and to distract, or the joint inserts or inter-facet spacers 1010 can be of substantially uniform thickness. The implants 1000 each comprise a joint insert or inter-facet spacer 1010 with an outer surface 1070 that interacts with the bone of the cervical facet joint 1001. On the upper implant 1000, the surface 1070 that interacts with the bone is the upper surface and on the lower implant 1000, the surface 1070 that interacts with the bone is the lower surface. As set forth above, each outer surface 1070 can comprise a bone ingrowth surface 1080 to create a porous surface and thereby promote bone ingrowth and fixation, without facet joint fusion and loss of mobility. In one preferred embodiment, the bone ingrowth surface 1080 can be created with plasma spray titanium, and/or with a coating of sintered beads. In an alternative preferred embodiment, the implant 1000 can have casted porous surfaces 1070, where the porous surface is integral to the implant 1000. In a further alternative preferred embodiment, the surfaces 1070 can be roughened in order to promote bone ingrowth into these defined surfaces of the implants 1000. In other preferred embodiments, the surfaces 1070 can be coated with materials that promote bone growth such as for example BMP, or structural materials such as HA, or other substances which promote growth of bone on other external surfaces 1070 of the implant 1000.

The implant 1000 can have a posterior alignment guide 1090. The posterior alignment guides 1090 of each implant 1000 can be continuous with the joint inserts or inter-facet spacers 1010. The posterior alignment guides substantially conform to the bone of the vertebrae when the joint inserts or inter-facet spacers 1010 are inserted into the cervical facet joint 1001. The posterior alignment guides 1090 are used to align the implants 1000 so that the joint inserts 1010 contact each other and not the bones of the cervical facet joint 1001 when the joint inserts 1010 or inter-facet spacers are positioned in the cervical facet joint 1001.

FIG. 14 depicts a further embodiment 1100 of the implant of the present invention. In this embodiment 1100, the joint inserts 1110 of two implants 1100 are inserted into the cervical facet joint 1101. Each of the joint inserts or inter-facet spacers 1110 is continuous with a cervical facet joint extender or facet-extending surface 1192. The bone contacting surfaces 1170 of the joint inserts or inter-facet spacers 1110 are continuous with, and at an angle to, the bone contacting surfaces 1193 of the cervical facet joint extenders 1192, so that the cervical facet joint extenders 1192 conform to the bones of the vertebrae exterior to the cervical facet joint 1101. The conformity of the cervical facet joint extenders 1192 is achieved for example by forming the cervical facet joint extenders 1192 so that when the join inserts or inter-facet spacers 1110 are positioned, the cervical facet joint extenders 1192 curve around the bone outsider the cervical facet joint 1101.

The cervical facet joint extenders have a second surface 1184 that is continuous with the joint articular surfaces 1182 of the joint inserts or inter-facet spacers 1110. The second surfaces 1184 extend the implant 1100 posteriorly to expand the joint articular surfaces 1182 and thereby to increase contact and stability of the spine at least in the region of the implants 1100. It is to be understood that such facet joint extenders 1192 can be added to the other embodiments of the invention described and depicted herein.

The embodiment depicted in FIG. 15 shows two implants 1200 positioned in a cervical facet joint 1201, having bony ingrowth surfaces as one preferred method of fixation, and using screws as another preferred method of fixation. In this embodiment, each of two implants 1200 has a joint insert or inter-facet spacer 1210 positioned in a cervical facet joint 1201. As described above, the joint inserts or inter-facet spacers 1210 can be wedge-shaped as needed to restore anatomical curvature of the cervical spine and to distract, or the joint inserts or inter-facet spacers 1210 can be of substantially uniform thickness. The implants 1200 each comprise a joint insert 1210 with an outer surface 1270 that interacts with the bone of the cervical facet joint 1001. On the upper implant 1200, the surface 1270 that interacts with the bone is the upper surface and on the lower implant 1200, the surface 1270 that interacts with the bone is the lower surface. As set forth above, each outer surface 1270 can comprise a bone ingrowth surface 1280 to create a porous surface and thereby promote bone ingrowth and fixation. In one preferred embodiment, the bone ingrowth surface 1280 can be created with plasma spray titanium, and/or with a coating of sintered beads. In an alternative preferred embodiment, the implant 1200 can have casted porous surfaces 1270, where the porous surface is integral to the implant 1200. In a further alternative embodiment, the surfaces 1270 can be roughened in order to promote bone ingrowth into these defined surfaces of the implants 1200. In other preferred embodiments, the surfaces 1270 can be coated with materials that promote bone growth such as for example BMP, or structural materials such as HA, or other substances which promote growth of bone on other external surfaces 1270 of the implant 1200.

Screw fixation or other appropriate fixation also can be used with implants 1200 for fixation in the cervical facet joint 1201. The joint insert or inter-facet spacer 1210 is continuous with a posterior sheath 1220 bent at an angle from the joint insert or inter-facet spacer 1210 to align substantially parallel with the bone, preferably the lateral mass or lamina. The posterior sheath 1220 can have a bore 1230 which can accept a bone screw 1240, preferably into the lateral mass or lamina. Alternatively, the bore 1230 can accept any other appropriate and/or equivalent fixation means for fixing the embodiment 1200 to the spine.

FIG. 16 depicts a further preferred embodiment of the present invention. In this embodiment 1300, two joint inserts or inter-facet spacers 1310 are positioned in the cervical facet joint 1301. The joint inserts each have outer surfaces 1370 that interact with the bone of the vertebrae forming the cervical facet joint. These outer surfaces 1370 of the embodiment 1300 can be treated to become bone ingrowth surfaces 1380, which bone ingrowth surfaces 1380 contribute to stabilizing the two joint inserts or inter-facet spacers 1310 of the implant 1300. In one preferred embodiment, the bone ingrowth surface 1380 can be created with plasma spray titanium, and/or with a coating of sintered beads. In an alternative preferred embodiment, the implant 1300 can have casted porous surfaces 1370, where the porous surface is integral to the implant 1300. In a further alternative embodiment, the surfaces 1370 can be roughened in order to promote bone ingrowth into these defined surfaces of the implants 1300. In other preferred embodiments, the surfaces 1370 can be coated with materials that promote bone growth such as for example BMP, or structural materials such as HA, or other substances which promote growth of bone on other external surfaces 1370 of the implant 1300. This fixation stabilizes the implant 1300 in the facet joint without fusing the joint, and thus the implant preserves joint mobility, while accomplishing distraction and increasing foraminal dimension.

Also shown in FIG. 16 are articular inner surfaces 1382 of the implants 1300. These surfaces can be formed from a metal and polyethylene, the material allowing flexibility and providing for forward bending/flexion and backward extension of the cervical spine. The embodiment 1300 of FIG. 16 can be made in at least two configurations. The first configuration includes a flexible spacer 1382 made, by way of example, using polyethylene or other suitable, flexible implant material. The flexible spacer 1382 can be permanently affixed to the upper and lower joint insert or inter-facet spacer 1310. The spacer 1382 can be flat or wedge-shaped or have any other shape that would correct the curvature of the spine. In other configurations, the spacer 1382 can be affixed to only the upper insert or inter-facet spacer 1310 or to only the lower insert 1310. Alternatively, a spacer 1382 can be affixed to each of an upper insert or inter-facet spacer 1310 and a lower insert or inter-facet spacer 1310 with the upper insert or inter-facet spacer 1310 and the lower insert or inter-facet spacer 1310 being separate units.

FIG. 17 shows a further preferred embodiment of the implant of the present invention. In this embodiment 1400, the implant has a roller 1496 mounted on a joint insert or inter-facet spacer 1410, the roller being a further means of preserving joint mobility while accomplishing distraction. Both the roller 1496 and the joint insert or inter-facet spacer 1410 are positioned in the cervical facet joint 1401. The joint insert or inter-facet spacer 1410 as in other embodiments has a bone-facing surface 1470 and joint articular surface 1482. The bone-facing surface 1470 can interact with the lower bone of the cervical facet joint 1401. Alternatively, the bone-facing surface can interact with the upper bone of the cervical facet joint 1401. Between the bone-facing surface 1470 and the joint articular surface 1482 is an axis about which the roller 1496 can rotate. The roller 1496 rotates in a cavity in the joint insert 1410, and interacts with the top bone of the cervical facet joint 1401. Alternatively, where the bone-facing surface 1470 of the joint insert or inter-facet spacer 1410 interacts with the top bone of the cervical facet joint 1401, the roller 1496 rotates in a cavity in the joint insert or inter-facet spacer 1410 and interacts with the lower bone of the cervical facet joint 1401. The rotation of the roller 1496 allows flexion and extension of the cervical spine. Alternatively, a roller such as roller 1496 can be secured to an upper and a lower insert such as inserts 410 in FIG. 7. As depicted in FIG. 18, a plurality of rollers 1496 also is possible.

FIG. 19 depicts a further embodiment of the implant of the present invention. In this embodiment, two implants 1500 are implanted in the cervical facet joint 1501. Screw fixation or other appropriate fixation is used with implants 1500 for fixation in the cervical facet joint 1501. The joint insert or inter-facet spacer 1510 is continuous with a posterior sheath 1520 bent at an angle from the joint insert or inter-facet spacer 1510 to align substantially parallel with the bone, preferably the lateral mass or lamina. The posterior sheath 1520 of each implant 1500 can have a bore 1530 which can accept a bone screw 1540, preferably into the lateral mass or lamina. Alternatively, the bore 1530 can accept any other appropriate and/or equivalent fixation means for fixing the embodiment 1500 to the spine. The head of the screw 1540 in each posterior sheath 1520 of each implant 1500 has a groove 1598 or other mechanism for retaining an elastic band 1597. The elastic band 1597 is looped around each of the two screws 1540 to restrain movement of the cervical spine without eliminating facet joint mobility. The band 1597 preferably can restrain flexion and lateral movement. The elastic band 1597 can be made of a biocompatible, flexible material.

FIG. 20 shows an alternative to use of an elastic band as in FIG. 19. In the embodiment in FIG. 20, the elastic band is replaced with a spring restraint 1699, which extends between the heads of two screws 1640, one screw fixing each of two implants 1600 in the cervical facet joint 1601.

FIG. 21 shows another alternative to using an elastic band and/or a spring as in FIG. 19 or 20. In FIG. 21, magnets 1795 are used for restraint between the two screws 1740. The magnet 1795 can either be comprised of two opposing magnetic fields or two of the same magnetic fields to operate to restrain movement. The head of one of the two screws 1740 is magnetized, and the head of the other screw 1740 is magnetized with either the same or opposite field. If the magnets 1795 have the same polarity, the magnets 1795 repel each other and thus limit extension. If the magnets 1795 have opposite polarities, the magnets 1795 attract each other and thus limit flexion and lateral movement.

FIGS. 22A-24B, depict a further embodiment 1800 of the implant of the present invention. In this embodiment, a facet joint spacer (or insert) or inter-facet spacer (or insert) 1810 is connected with a lateral mass plate (also referred to as an anchoring palte) 1820 with a hinge 1822. The hinge 1822 allows the lateral mass plate 1820 to bend at a wide range of angles relative to the artificial facet joint and preferably at an angle of more than 90 degrees, and this flexibility facilitates positioning and insertion of the facet joint spacer (or insert) or inter-facet spacer (or insert) 1810 into a patient's facet joint, the anatomy of which can be highly variable among individuals. This characteristic also applies to embodiments described below, which have a hinge or which are otherwise enabled to bend by some equivalent structure or material property. The hinge 1822 further facilitates customizing the anchoring of the implant, i.e., the positioning of a fixation device. The hinge enables positioning of the lateral mass plate 1820 to conform to a patient's cervical spinal anatomy, and the lateral mass plate 1820 accepts a fixation device to penetrate the bone. The facet joint spacer (or insert) or inter-facet spacer (or insert) 1810 can be curved or rounded at a distal end 1812 (FIG. 23A), and convex or dome-shaped on a superior surface 1813 to approximate the shape of the bone inside the facet joint. The inferior surface 1815 can be flat or planar. Alternatively, the inferior surface 1815 can be concave. As another alternative, the inferior surface 1815 can be convex.

The lateral mass plate 1820, when implanted in the spine, is positioned outside the facet joint, preferably against the lateral mass or against the lamina. The lateral mass plate 1820 has a bore 1830 therethrough. The bore 1830 can accept a bone screw 1840, also referred to as a lateral mass screw, to secure the lateral mass plate 1820 preferably to the lateral mass or alternatively to another part of the spine, and thus to anchor the implant. The lateral mass screw 1840 preferably has a hexagonal head to accept an appropriately-shaped wrench. As described below, the head accepts a compatible probe 1826 from a locking plate 1824.

The locking plate 1824 includes a keel 1828 with a wedge shaped distal end to anchor the implant, preferably in the lateral mass or in the lamina, outside the facet joint and to prevent rotation of the lateral mass plate 1820 and the locking plate 1824. The keel 1828 aligns with a groove 1823 through an edge of the lateral mass plate 1820 to guide and align the keel 1828 as the keel 1828 cuts into a vertebra.

As noted above, the locking plate 1824 includes a probe 1826 that fits against the head of the lateral mass screw 1840. The locking plate further includes a bore 1831 that can accept a machine screw (not shown) which passes through to an aligned bore 1829 in the lateral mass plate 1820 to hold the locking plate 1824 and the lateral mass plate 1820 together without rotational displacement relative to each other. The locking plate 1824 thus serves at least two functions: (1) maintaining the position of the lateral mass screw 1840 with the probe 1826, so that the screw 1840 does not back out; and (2) preventing rotation of the implant with the keel 1828 and machine screw relative to the cervical vertebra or other vertebrae.

It is to be understood that other mechanisms can be used to lock the locking plate 1824 to the lateral mass plate 1820. For example, the locking plate can include a probe with barbs that can be inserted into a port in the lateral mass plate. The barbs can become engaged in ribs that define the side walls of the port in the lateral mass plate.

In the preferred embodiment depicted in FIGS. 25A, 25B, the lateral mass plate 1920 includes a recessed area 1922 for receiving the locking plate 1924 so that the locking plate 1924 is flush with the upper surface 1925 of the lateral mass plate 1920 when the probe 1926 is urged against the lateral mass screw 1940 and the keel 1928 is inserted into the lateral mass or the lamina of the vertebra. In the preferred embodiment depicted in FIGS. 25A, 25B, the shape and contours of the facet joint spacer (or insert) or inter-facet spacer (or insert) 1910 can facilitate insertion of the facet joint spacer (or insert) or inter-facet spacer (or insert) 1910 into the cervical facet joint. In this embodiment, the facet joint spacer (or insert) or inter-facet spacer (or insert) 1910 has a rounded distal end 1912. The distal end 1912 is tapered in thickness to facilitate insertion. The tapered distal end 1912 meets and is continuous with a proximal mid-section 1916 which, in this preferred embodiment, has a uniform thickness, and is connected flexibly, preferably with a hinge 1922, to the lateral mass plate 1920, as described above. The facet joint spacer (or insert) or inter-facet spacer (or insert) 1910, with its proximal mid-section 1916 and tapered distal end 1912, is curved downward, causing a superior surface 1913 of the facet joint spacer (or insert) or inter-facet spacer (or insert) 1910 to be curved. The curve can cause the superior surface 1913 to be convex, and the convexity can vary among different implants 1900 to suit the anatomical structure of the cervical facet joint(s) of a patient. An inferior surface 1915 accordingly can be preferably concave, flat, or convex. The curved shape of the implant can fit the shape of a cervical facet joint, which is comprised of an inferior facet of an upper vertebra and a superior facet of a lower adjacent vertebra. The convex shape of the superior surface 1913 of the facet joint spacer (or insert) or inter-facet spacer (or insert) 1910 fits with a concave shape of the inferior facet of the upper cervical vertebrae. The concave shape of the inferior surface 1915 of the facet joint spacer (or insert) or inter-facet spacer (or insert) 1910 fits with the convex shape of the superior facet of the cervical vertebrae. The degree of convexity and concavity of the facet joint spacer (or insert) or inter-facet spacer (or insert) inferior and superior surfaces can be varied to fit a patient's anatomy and the particular pairing of adjacent cervical vertebrae to be treated. For example, a less-curved facet joint spacer (or insert) or inter-facet spacer (or insert) 1910 can be used where the patient's cervical spinal anatomy is sized (as described below) and found to have less convexity and concavity of the articular facets. Generally for the same level the input for the right and left facet joint will be similarly shaped. It is expected that the similarity of shape of the facet joint spacer (or insert) or inter-facet spacer (or insert) and the smooth, flush surfaces will allow distraction of the facet joint without loss of mobility or damage to the bones of the cervical spine. Further, and preferably, the width of the mid-section 1916 is from 1.5 mm to 2.5 mm.

Except as otherwise noted above, the embodiment shown in FIGS. 22A-24B is similar to the embodiment shown in FIGS. 25A, 25B. Accordingly the remaining elements on the 1900 series of element numbers is preferably substantially similar to the described elements in the 1800 series of element numbers, as set forth above. Thus, by way of example, elements 1923, 1928, 1929 and 1930 are similar, respective elements 1823, 1828, 1829 and 1830.

FIG. 30 is a flow chart of the method of insertion of an implant of the invention. The embodiment 1800 or 1900 of the present invention preferably is inserted in the following manner (only elements of the embodiment 1800 will be set forth herein, for purposes of the written description of a method of the invention). First the facet joint is accessed. A sizing tool 2200 (see FIGS. 29A-C) can be inserted to select the appropriate size of an implant of the invention for positioning in the cervical facet joint. This step may be repeated as necessary with, if desired, different sizes of the tool 2200 until the appropriate size is determined. This sizing step also distracts the facet joint and surrounding tissue in order to facilitate insertion of the implant. Then, the natural or artificial facet joint spacer or inter-facet spacer 1810 is urged between the facets into the facet joint. The facet itself is somewhat shaped like a ball and socket joint. Accordingly, in order to accommodate this shape, the natural or artificial joint spacer or inter-facet spacer 1810 can have a rounded leading edge shaped like a wedge or tissue expander to cause distraction of the facet joint as the natural or natural or artificial facet joint spacer or inter-facet spacer spacer or inter-facet spacer is urged into the facet joint of the spine. The natural or artificial facet joint spacer or inter-facet spacer 1810 also includes the convex surface 1813 in order to more fully accommodate the shape of the facet joint of the spine. However, as set forth above and as depicted in FIG. 25B, it is possible in the alternative to have a curve-shaped natural or artificial facet joint spacer or inter-facet spacer 1910 with a convex superior surface 1913 and a concave inferior surface 1915, the distal end 1912 tapering to facilitate insertion, while the remainder of the natural or artificial facet joint spacer or inter-facet spacer 1910, (i.e., the proximal section 1916) has a uniform thickness.

Once the natural or artificial joint spacer or inter-facet spacer 1810 is positioned, the lateral mass plate 1820 is pivoted downward about the hinge 1822 adjacent to the vertebrae and preferably to the lateral mass or to the lamina. Thus the lateral mass plate 1820 may be disposed at an angle relative to the natural or artificial facet joint spacer or inter-facet spacer 1810 for a representative spine configuration. It is to be understood that as this embodiment is hinged the final position of the lateral mass plate 1820 relative to the natural or artificial facet joint spacer or inter-facet spacer 1810 will depend on the actual spine configuration. It is to be understood that embodiments of the invention can be made without a hinge, as long as the connection between the natural or artificial facet joint spacer or inter-facet spacer and the lateral mass plate is flexible enough to allow the lateral mass plate to be bent relative to the natural or artificial facet joint spacer or inter-facet spacer in order to fit the anatomy of the patient. Once the lateral mass plate 1820 is positioned, or prior to the positioning of the lateral mass plate 1820, a bore can be drilled in the bone to accommodate the bone screw 1824. Alternatively the screw 1824 can be self-tapping. The screw is then placed through the bore 1830 and secured to the bone, preferably the lateral mass or the lamina, thereby holding the natural or artificial facet joint spacer or inter-facet spacer 1810 in place. In order to lock the bone screw 1824 in place and to lock the position of the natural or artificial facet joint spacer or inter-facet spacer 1810 and the lateral mass plate 1820 in place, the locking plate 1824 is positioned over the lateral mass plate 1820. So positioned, the probe 1826 is positioned through the bore 1830 and against the head of the bone screw to keep the bone screw from moving. The keel 1828, having a sharp chisel-shaped end, preferably can self-cut a groove in the bone so that the keel 1828 is locked into the bone as the keel 1828 is aligned by, and received in, a groove 1831 of the lateral mass plate 1820. Alternatively, a groove can be pre-cut in the bone to receive the keel 1828. As this occurs the bore 1829 of the locking plate 1824 aligns with the threaded bore 1831 of the lateral mass plate 1820 and a machine screw can be inserted to lock the locking plate relative to the lateral mass plate. This locking prevents the lateral mass plate 1820 and the natural or artificial facet joint spacer or inter-facet spacer 1810 from rotating and, as previously indicated, prevents the bone screw 1840 from backing out from the vertebra. Preferably the implant is between the C5 and C6 vertebrae level, or the C6 and C7 vertebrae level. It is noted that two implants preferably will be implanted at each level between vertebrae. That is, an implant 1800 will be placed in a right facet joint and also in a left facet joint when viewed from a posterior view point. This procedure can be used to increase or distract the foraminal area or dimension of the spine in an extension or in neutral position (without having a deleterious effect on cervical lordosis) and reduce the pressure on the nerves and blood vessels. At the same time this procedure preserves mobility of the facet joint.

FIGS. 26A-27B show a further embodiment of the implant of the invention, with the embodiment 2000 implanted in the cervical spine as depicted in FIGS. 27A and 27B. The implant 2000 comprises a first natural or artificial facet joint spacer or inter-facet spacer 2010 and a second natural or artificial facet joint spacer or inter-facet spacer 2010. Each natural or artificial facet joint spacer or inter-facet spacer can have a distal end 2012 that is tapered or wedge-shaped in a way that facilitates insertion into the cervical facet joints on both sides of two adjacent cervical vertebrae at the same level. The natural or artificial facet joint spacers or inter-facet spacers further can be dome-shaped, or convex on a superior surface 2013, to approximate the shape of the cervical facets of the cervical facet joints.

The first and second natural or artificial facet joint spacer or inter-facet spacer s 2010 are bridged together by a collar 2015. The collar 2015 passes between the spinous processes of the adjacent cervical vertebrae. As can be seen in FIG. 26B, the implant can preferably be “V” shaped or “boomerang” shaped. The entire implant 2000 or the collar 2015 of the implant can be made of a flexible material such as titanium, so that it is possible to bend the collar 2015 so that it conforms preferably to the shape of the lateral mass or the lamina of the cervical vertebrae of the patient and thereby holds the implant in place with the natural or artificial facet joint spacer or inter-facet spacer 2010 inserted in the cervical facet joints. Bores 2029 are preferably are provided through implant 2000 adjacent to the natural or artificial facet joint spacer or inter-facet spacer 2010 respectively. These bores 2029 can receive bone screws to position the implant 2000 against the lateral mass or the lamina as shown in FIGS. 27A, 27B. The description of the embodiment 2100, in FIGS. 28A, 28B provide further details concerning the method of affixing the implant 2000 to the vertebrae. The implant 2100 also can be made of PEEK or other materials as described herein. Embodiment 2000 (the “boomerang” shape depicted in FIG. 27B) further can have a locking plate as, for example, the locking plate 1824 in FIG. 22A. The locking plate for embodiment 2000 (not shown) can have the same features as locking plate 1824, that is: (1) a probe 1826 that interacts with the bone screws to prevent the bone screws from backing out of the bone, the likely consequence of which would be displacement of the implant 2000; and (2) a keel 1828 with a chisel end to embed in the bone and thus to prevent rotational displacement of the implant. However, given the collar 2015 configuration of embodiment 2000, a chisel may not serve the same purpose as with the embodiments set forth above, which lack a collar stabilized by two bone screws. Therefore, a locking plate on embodiment 2000 can be provided without a keel.

FIGS. 28A and 28B depict a further embodiment of the implant of the invention 2100. In this embodiment 2100, the collar 2115 can be made of a flexible material such as titanium, of a substantially inflexible material, or of other materials described herein. Substantial flexibility can also be derived from connecting a first natural or artificial facet joint spacer or inter-facet spacer 2110 with the collar 2115 using a first hinge 2117, and connecting a second natural or artificial facet joint spacer or inter-facet spacer 2110 with the collar 2115 using a second hinge 2117. Using the first hinge 2117 and the second hinge 2117, the collar 2115 can be pivoted downward to conform to a particular patient's cervical spinal anatomy. In other words, the degree of pivoting will vary among different patients, and the first hinge 2117 and second hinge 2117 allow the implant 2100 to accommodate the variance.

In the hinged embodiment 2100, and similar to the embodiment 2000, the collar 2115 can have a first bore 2129 inferior to the first hinge 2117, and a second bore 2129 inferior to the second hinge 2117. A first bone screw penetrates the first bore 2130 and into the lateral mass or the lamina, and the second bone screw penetrates the second bore 2130 and into the lateral mass or the lamina, the first and second bone screws serving to anchor the implant. A bore, preferably in the lateral mass, can be drilled for the first bone screw and for the second bone screw. Alternatively, the bone screws can be self-tapping. A first locking plate similar to the plate 1924 (FIG. 25A) can be secured about the head of the first bone screw and a second locking plate can be secured about the head of the second bone screw to prevent displacement of the first and second bone screws 2140. The first locking plate can block the first bone screw with a probe and the second locking plate can block to the second bone screw with a probe.

It should be noted that embodiments 2000 and 2100 also can be configured for accommodating treatment of cervical spinal stenosis and other cervical spine ailments where only a single cervical facet joint between adjacent vertebrae requires an implant, i.e., where treatment is limited to one lateral facet joint. In that case, the collar 2015, 2115 extends medially without extending further to join a second natural or artificial facet joint spacer or inter-facet spacer 2010, 2110. For the hinged embodiment 2100, the implant comprises a single hinge 2117, and the collar 2115 has only one bore 2129 to accept one bone screw to secure the implant 2100.

FIGS. 29A-E, depict a sizing and distracting tool 2200 of the invention. Sizing tool 2200 has a handle 2203 and a distal head 2210 that is shaped as a natural or artificial facet joint spacer or inter-facet spacer (e.g., 1810) of an implant of the invention. That is, the head 2210 preferably will have essentially the same features as the natural or artificial facet joint spacer or inter-facet spacer 1810, but the dimensions of the head 2210 will vary from one tool 2200 to the next, in order to be able to use different versions of the sizing tool 2200 to determine the dimensions of the cervical facet joint that is to be treated and then to select an appropriately-sized implant. The head 2210 preferably can be used to distract the facet joint prior to the step of implanting the implant in the facet joint. In this regard, the head 2210 is rounded at the most distal point 2212, and can be a tapered to facilitate insertion into a cervical facet joint. The head 2210 also can have a slightly convex superior surface 2213, the degree of convexity varying among different sizing tools 2200 in order to determine the desired degree of convexity of an implant to be implanted in the cervical facet joint. The head 2210 may have a uniform thickness along a proximal mid-section 2216. Accordingly, the inferior surface 2215 preferably can be concave. Alternatively, the proximal mid-section 2212 may be convex on the superior surface 1813 without being uniform in thickness. Thus, the inferior surface 2215 can be flat or planar. The head also can be curved.

The head 2210 has a stop 2218 to prevent over-insertion of the head 2210 of the sizing tool 2200 into the facet joint. The stop 2218 can be a ridge that separates the head 2210 from the handle 2203. Alternatively, the stop 2218 can be any structure that prevents insertion beyond the stop 2218, including pegs, teeth, and the like.

Different sizing tools 2200 covering a range of dimensions of the head 2210 can be inserted successively into a cervical facet joint to select the appropriate size of an implant to position in the cervical spine, with the appropriate convexity and concavity of the natural or artificial facet joint spacer or inter-facet spacer. Each preferably larger head also can be used to distract the facet joint.

FIG. 31A depicts a posterior view of a further embodiment 2300 of the implant of the invention. Embodiment 2300, as well as all of the embodiments herein, can benefit from some or all of the advantages described herein with regard to the other embodiments described herein. Further, FIG. 31A, embodiment 2300 has a natural or artificial facet joint spacer or inter-facet spacer 2310 that can have a tapered or thinned distal end 2312 so that the distal end 2312 facilitates insertion of the natural or artificial facet joint spacer or inter-facet spacer 2310 into a cervical facet joint. The distal end 2312 can be rounded, as seen in the plan view of FIG. 31A, in order to conform to the roundness of the facet joint. The natural or artificial facet joint spacer or inter-facet spacer 2310 further can be curved so that a superior surface 2313 of the natural or artificial facet joint spacer or inter-facet spacer 2310 is convex, and an inferior surface 2315 is concave, to approximate the natural shape of the cervical facet joint that is to receive the implant 2300. The curve can have a uniform thickness, or it can have a varied thickness. Further, the lateral edges of the natural or artificial facet joint spacer or inter-facet spacer 2310 are curved or rounded, for distribution of load-bearing stress. As with other embodiments described herein, the natural or artificial facet joint spacer or inter-facet spacer 2310 also can be made of a flexible, biocompatible material, such as PEEK, to maintain joint mobility and flexibility.

The natural or artificial facet joint spacer or inter-facet spacer 2310 is connected flexibly with a lateral mass plate 2320, the flexible connection preferably being a hinge 2322. As seen in the plan view of FIG. 31A, the implant 2300 is substantially hour-glass shaped. This shape, as well as the shape of FIG. 32, will be discussed further below. The hinge 2322 is narrower than the natural or artificial facet joint spacer or inter-facet spacer 2310, with the hinge 2322 sitting at substantially the isthmus 2317 between the natural or artificial facet joint spacer or inter-facet spacer 2310 and the lateral mass plate 2320. The curved edges, or fillets, about the hinge 2322 serve to distribute more evenly the load-bearing stress on the implant 2300, and thus prevent concentrating the stress about the edges.

The hinge 2322 allows the implant 2300 to bend at the hinge 2322, bringing a lateral mass plate 2320 adjacent to the lateral mass and/or lamina of the patient's spine, and to conform to a particular patient's anatomy. The lateral mass plate 2320 is made of a biocompatible flexible material, preferably titanium or any other biocompatible flexible material as described herein, for example PEEK, that will support the use of bone screws and other hardware, as described below. The lateral mass plate 2320 bends downward at the hinge 2322 over a wide range of angles relative to the natural or artificial facet joint spacer or inter-facet spacer 2310, and preferably at an angle of more than 90 degrees, and this flexibility facilitates positioning and insertion of the natural or artificial facet joint spacer or inter-facet spacer. This flexibility of the lateral mass plate 2320 relative to the natural or artificial facet joint spacer or inter-facet spacer 2310 further facilitates positioning of the lateral mass plate relative to the lateral mass and/or the lamina of the patient's spine. Once the lateral mass plate 2320 is positioned adjacent to the bone, preferably the lateral mass of a cervical vertebra, a first bone screw, such as bone screw 1840, can be inserted through a first bore 2330 through the lateral mass plate 2320 and embedded into the bone of the lateral mass of the cervical vertebra.

The lateral mass plate 2320 further comprises a second bore 2329 which is preferably positioned medially, relative to the first bore 2330. Thus, viewing the implant from a posterior perspective as in FIG. 31A, the second bore 2329 in the lateral mass plate 2320 can be positioned either to the left or to the right of the first bore 2330. The position of the second bore 2329 will depend upon whether the implant 2300 is intended to be inserted into a cervical facet joint on the left or right side of a patient. Specifically, an implant 2300 to be inserted into a right-side cervical facet joint (i.e., the patient's rights side) will have a second bore 2329 positioned to the left of the first bore 2330 as in FIG. 31A, when implant 2300 is viewed from a posterior perspective, while an implant 2300 to be inserted into a left-side cervical facet joint will have a second bore 2329 positioned to the right of the first bore 2330, when implant 2300 is viewed from a posterior perspective.

The second bore 2329 through the lateral mass plate 2320 is adapted to accept a second screw 2390 (FIG. 31B), which preferably is a locking screw with a chisel point 2391. The locking screw 2390 is received by the second bore 2329 and the chisel point 2391 self-cuts a bore into the bone. The locking screw 2390 preferably is inserted through the second bore 2329 and embedded in the bone, after the bone screw is embedded in the bone through the first bore 2330. The position of the second bore 2329, i.e., medial to the first bore 2330, positions the locking screw 2390 so that it embeds in stronger bone tissue than if the second bore 2329 were located more laterally. The locking screw, in combination with the bone screw, prevents rotational and/or backward displacement of the implant 2300. As the locking screw 2390 is received by the second bore 2329, the head 2392 of the locking screw 2390 aligns with the head of the first bone screw in the first bore 2330, blocking the head of the first bone screw to prevent the first bone screw from backing out of the bone of the vertebra and the first bore 2330.

FIG. 32 depicts a further embodiment 2400 of the implant of the invention, from a posterior view. Embodiment 2400 is adapted to be implanted in a manner that preserves the anatomy of the cervical facet joint, in particular, the soft tissues around the cervical facet joint, including the joint capsule.

Implant 2400, like implant 2300 and other implants disclosed above, has a natural or artificial facet joint spacer or inter-facet spacer 2410, flexibly connected, preferably by a hinge 2422, to a lateral mass plate 2420. As can be seen in FIG. 32, the implant 2400 including the natural or artificial facet joint spacer or inter-facet spacer 2410 and the hinge 2422 is substantially “P” shaped. As explained below, its “P” shape assists in the insertion of the implant 2400 into the facet joint with most of the facet capsule and facet capsule ligament and other soft tissue associated with the facet joint still left intact. The natural or artificial facet joint spacer or inter-facet spacer, as above for implant 2300 and the other implants disclosed above, can have a superior surface 2413 of the natural or artificial facet joint spacer or inter-facet spacer 2410 that is convex, and an inferior surface 2415 that is concave, or any appropriate shaping to approximate the natural shape of the cervical facet joint that is to receive the implant 2400. The thickness of the natural or artificial facet joint spacer or inter-facet spacer 2410 can be uniform, or varied. The natural or artificial facet joint spacer or inter-facet spacer 2410 also can be made of a flexible, biocompatible material, such as PEEK, to maintain joint mobility and flexibility. The hinge 2422 can have smooth, rounded edges, for distribution of load stress, as disclosed above. Other features and advantages of the other embodiments can be, if desired, incorporated into the design of the embodiment of FIG. 32. For example, the natural or artificial facet joint spacer or inter-facet spacer 2410 further can have a tapered or thinned edge 2412 so that the edge 2412 facilitates insertion of the natural or artificial facet joint spacer or inter-facet spacer 2410 into a cervical facet joint. The edge 2412 can be curved. In this embodiment 2400, however, the thinned edge 2412 of the natural or artificial facet joint spacer or inter-facet spacer 2410 preferably is not at the distal end of the natural or artificial facet joint spacer or inter-facet spacer 2400 as is the thinned edge 2312 of the natural or artificial facet joint spacer or inter-facet spacer 2300; rather, the thinned edge 2412 preferably is positioned laterally, toward the hinge 2422 of the implant 2400. The thinned edge 2412 coincides substantially with a lateral curvature 2440 of the natural or artificial facet joint spacer or inter-facet spacer 2410, which is pronounced relative to the curvature on the medial side of the implant 2400, i.e., a “P” shape. In other words, the curved part of the head of the “P” 2440 corresponds to the thinned edge 2412, and serves as the leading edge of the implant 2400 to begin insertion of the natural or artificial facet joint spacer or inter-facet spacer 2410 into a cervical facet joint, preferably through an incision in the soft tissue of the facet joint. The “P” shape narrows at isthmus 2417 where the natural or artificial facet joint spacer or inter-facet spacer 2410 that is joined by the hinge 2422 with the lateral mass plate 2420. The smooth or rounded edges or fillets serve to distribute stresses on the implant 2400. The above described “P” shape of implant 2400 allows the implant 2400 to be pivoted into place into a facet joint as described below. The thinned edge 2412 and leading lateral curvature 2440 of the natural or artificial facet joint spacer or inter-facet spacer 2410 are adapted to facilitate urging implant 2400 into the cervical facet joint, through the incision in the joint capsule. The implant 2400 then is pivoted into position so that the lateral mass plate 2420 can be bent downward, relative to the natural or artificial facet joint spacer or inter-facet spacer 2410, to align with and lie adjacent to the lateral mass and/or the lamina. The lateral mass plate 2420 is then fastened to the bone.

The lateral mass plate 2420 of implant 2400, like the lateral mass plate for implant 2300, is flexibly connected, preferably by the smooth-edged hinge 2422, to the natural or artificial facet joint spacer or inter-facet spacer 2410 at the narrow lower part of the natural or artificial facet joint spacer or inter-facet spacer. The lateral mass plate 2420 is made of a biocompatible flexible material, preferably titanium or any other biocompatible flexible material such as PEEK that will support the use of bone screws and other hardware, as described below.

The lateral mass plate 2420 bends downward at a wide range of angles relative to the natural or artificial facet joint spacer or inter-facet spacer 2410, and preferably at an angle of more than 90 degrees. The flexibility of the lateral mass plate 2420 relative to the natural or artificial facet joint spacer or inter-facet spacer 2410 further facilitates positioning of the lateral mass plate 2420 relative to the lateral mass and/or the lamina of the patient's spine.

Like embodiment 2300, described above, the lateral mass plate 2420 has first bore 2430, which is adapted to receive a bone screw 2440, to help anchor implant 2400 in position. The lateral mass plate 2420 further includes a second bore 2429 adapted to be positioned medially, relative to the first bore 2430, as disclosed above for implant 2300. The position of the second bore 2429, when viewing implant 2400 from a posterior perspective (FIG. 32), will depend upon whether implant 2400 is intended to be implanted into a left-side or right-side cervical facet joint of a patient. Thus, implant 2400 with the second bore 2429 positioned to the left of the first bore 2430 is intended to be implanted in a right-side cervical facet joint of a patient, as depicted in FIG. 32, while an implant 2400 with a second bore 2429 positioned to the right of the first bore 2430 is intended to be implanted in a left-side cervical facet joint of a patient.

The second bore 2429 through the lateral mass plate 2420 is adapted to receive a second screw 2490 with head 2492, which preferably is a locking screw with a chisel point, such as screw 2390. The function and purpose of the bone screw disposed through bore 2430 and the locking screw disposed through bore 2429 are as described above with respect to the implant 2300.

The present invention further includes a method of implanting the implant 2400 (FIGS. 33A, 33B). To insert the natural or artificial facet joint spacer or inter-facet spacer 2410, a facet joint is accessed and an incision or a pair of incisions is made in the capsular ligament, the joint capsule, and the synovial membrane so that the thinned edge 2412 of the implant 2400 can be urged into the cervical facet joint through these tissues. The capsular ligament and the joint capsule and other soft tissues around the cervical facet joint are allowed to remain substantially intact, except for the small incision, and will be sutured and allowed to heal around the implant 2400. If desired, the cervical facet joint can be distracted prior to urging the curved section 2440 with the thinned edge 2412 of the natural or artificial facet joint spacer or inter-facet spacer 2410 into the cervical facet joint. Once the curved section 2440 of the natural or artificial facet joint spacer or inter-facet spacer 2410 with the thinned edge 2412 is urged into the cervical facet joint, implant 2400 is pivoted, preferably about 90 degrees, so that the second bore 2429 is placed medially relative to the first bore 2430. This allows the natural or artificial facet joint spacer or inter-facet spacer 2410 to be positioned in the facet joint. It is noted that the overall size, including the isthmus 2417, of the artificial fact joint 2410, as that of 2310, can be somewhat smaller than in prior embodiments to allow the natural or artificial facet joint spacer or inter-facet spacer to be positioned within the edges of the facet joint with the joint capsule substantially intact. The lateral mass plate 2420 then can be bent downward about the hinge 2422 into position adjacent the lateral mass or lamina of the spine of the patient, which position will depend upon the anatomy of an individual patient's cervical spine.

Once the lateral mass plate 2420 is positioned adjacent to the bone, preferably the lateral mass of a cervical vertebra, a first bone screw can be inserted through the first bore 2430 through the lateral mass plate 2420 and become embedded into the bone of the lateral mass of the cervical vertebra to anchor the implant 2400. After the bone screw is embedded, a locking screw is inserted through the second bore 2429 of the lateral mass plate 2420, the second bore 2429 medial to the first bore 2430. The locking screw has a chisel end that allows the locking screw to dig into the bone without use of a tool to pre-cut a bore. Alternatively, a bore can be pre-cut and a locking screw without a chisel end can be used. As the locking screw is embedded in the bone, the locking head of the locking screw is brought into proximity with the head of the bone screw to block its backward movement so that the implant 2400 remains anchored with the bone screw, i.e., so that the bone screw cannot back out of the bone. The embedded locking screw also serves to prevent rotational displacement of implant 2400, while blocking backward displacement of the first bone screw.

FIG. 34A depicts a posterior view of another embodiment 2500 of the implant of the invention. Embodiment 2500, as well as all of the embodiments herein, can benefit from some or all of the features and advantages with regard to the other embodiments described herein. As shown, embodiment 2500 has a natural or artificial facet joint spacer or inter-facet spacer 2510 that can have a tapered or thinned distal end 2512. The natural or artificial facet joint spacer or inter-facet spacer 2510 further can be curved so that a superior surface 2513 of the natural or artificial facet joint spacer or inter-facet spacer 2510 is convex, and an inferior surface 2515 is concave, to approximate the natural shape of the cervical facet joint that is to receive the implant 2500. In one embodiment, the inferior surface 2515 is substantially flat whereby the superior surface 2513 is convex (FIG. 34B). As shown in FIG. 34B, the convex superior surface 2513 tapers downward at an increased angle toward the inferior surface 2515 at the distal end 2512. This contour of the superior surface 2513 aids in smooth insertion of the natural or artificial facet joint spacer or inter-facet spacer 2510 into the facet joint. As with other embodiments described above, the natural or artificial facet joint spacer or inter-facet spacer 2510 also can be made of a flexible, biocompatible material, such as PEEK, to maintain joint mobility and flexibility.

The natural or artificial facet joint spacer or inter-facet spacer 2510 is connected flexibly with the lateral mass plate 2520, preferably with a hinge 2522. The hinge 2522 allows the natural or artificial facet joint spacer or inter-facet spacer 2510 and the lateral mass plate 2520 of the implant 2500 to bend with respect to one another between an extended position and a bent or folded position as discussed above. Once the lateral mass plate 2520 is positioned adjacent to the bone, preferably the lateral mass of a cervical vertebra, a first bone screw, such as bone screw 1840, can be inserted through a first bore 2530 through the lateral mass plate 2520 and embedded into the bone of the lateral mass of the cervical vertebra. In addition, once the lateral mass plate 2520 is secured with the first bone screw, a second bone screw can be inserted through a second bore 2529 in the lateral mass plate 2520, whereby the second bone screw would be embedded into the bone of the lateral mass of the cervical vertebra. Details of the first and second bores are discussed above.

The lateral mass plate 2520 is made of a biocompatible flexible material, preferably titanium or any other biocompatible flexible material as described herein, for example PEEK, that will support the use of bone screws and other hardware, as described below. The lateral mass plate 2520 bends downward about the hinge 2522 over a wide range of angles relative to the natural or artificial facet joint spacer or inter-facet spacer 2510. In another embodiment, any other type of interface between the natural or artificial facet joint spacer or inter-facet spacer 2510 and the lateral mass plate 2520 is contemplated (e.g. ball and socket joint). This flexibility facilitates positioning and insertion of the natural or artificial facet joint spacer or inter-facet spacer 2510.

FIG. 34B depicts a side view of the natural or artificial facet joint spacer or inter-facet spacer and lateral mass plate in accordance with one embodiment. As shown in FIG. 34B, the natural or artificial facet joint spacer or inter-facet spacer 2510 includes an hyper-extension tab 2522 in one embodiment. The hyper-extension tab 2522 prevents the natural or artificial facet joint spacer or inter-facet spacer 2510 as well as the lateral mass plate 2520 from moving in a direction beyond the extended position which is shown in FIGS. 34A and 35B. The lateral mass plate 2520 preferably includes a recess 2511 at the interface between the lateral mass plate 2520 and the natural or artificial facet joint spacer or inter-facet spacer 2510 which seats the tab 2522 in the extended position which is shown in FIG. 34A. When the natural or artificial facet joint spacer or inter-facet spacer 2510 is bent at an angle, the tab 2522 is not in contact with the recess 2511. However, the tab 2522 comes into contact with the recess 2511 when in the extended position, as shown in FIG. 34A. In addition, the tab 2522, when seated in the recess 2511, prevents the natural or artificial facet joint spacer or inter-facet spacer 2510 and lateral mass plate 2520 from moving beyond the extended position. This features aids in placing the implant into the facet joint as the implant is prevented from bending back beyond the extended position shown in FIG. 34B. This arrangement, however, allows the lateral mass plate 2520 to bend down to meet the spine when the natural or artificial facet joint spacer or inter-facet spacer 2510 is implanted in the facet joint.

As shown in FIG. 34A, the lateral mass plate 2520 preferably includes a third bore 2502 located near a rear edge, whereby the third bore 2052 preferably receives an engaging rod 2616 (FIG. 35B) of an implantation tool 2600 described below. The third bore 2502 preferably extends through the superior and inferior surfaces of the lateral mass plate, although not necessarily. Although the third bore 2502 is circular in shape, any other shape is contemplated which engages a correspondingly shaped engaging rod 2616 (FIG. 35B). The rear edge 2504 of the lateral mass plate 2520 can be engaged by the engagement head 2606 (FIG. 35B) of the implantation tool 2600 as described below.

In addition, the lateral mass plate 2520 preferably includes one or more winged protrusions, such as tabs, winglets or ears, 2508 which protrude from the side edges of the lateral mass plate 2520. FIG. 34A illustrates the implant 2500 having two winged protrusions 2508. The protrusions 2508 serve as guides to successfully couple the implant 2500 to the implantation tool 2600. In addition, the protrusions act as an engaging mechanism which secures the implant 2500 to the tool 2600. It should be noted that the winged protrusions 2508 are preferred and the implant 2500 can be configured in any other appropriate design to ensure that the implant 2500 is able to be effectively guided and secured to the implantation tool 2600.

FIG. 35A depicts an implantation tool in accordance with one embodiment of the present invention. As shown in FIG. 35A, the tool 2600 preferably includes a handle 2602 having a proximal end and a distal end. The tool 2600 includes an actuating switch 2608 as well as a shaft 2604 extending from the distal end of the handle 2602. As shown in FIG. 35A, the shaft 2604 preferably extends axially with the handle 2602, although the shaft 2604 may be at an angle with respect to the handle 2602. Extending from the shaft 2604 is an engagement head 2606, whereby the engagement head is preferably oriented at an angle with respect to the shaft 2604 and/or the handle 2602. The angle of the head 2606 relative to the shaft 2604 aids the surgeon in the process of implanting the implant 2500 in the spine. This angle allows the surgeon to slip the natural or artificial facet joint spacer or inter-facet spacer 2510 into the facet joint with the tool 2600 preferably about a right angle to the spine. Preferably the head is at an angle between 45 and 90 degrees relative to the handle 2604. However, other angles are contemplated.

Referring to FIG. 35B, the engagement head 2606 preferably has a forked configuration and includes a pair of side walls 2610, an engagement seat 2612 as well as a receiving space 2618 which is defined as the area between the side walls 2610 and the seat 2612. The engagement head 2606 preferably includes a retractable engaging rod 2616 which extends partially into the receiving space 2618. The side walls 2610 each have an inner side which includes a slot 2612 whereby the slots 2612 face the receiving space 2618. The slots 2612 are dimensioned to slidably receive the wing protrusions 2508 of the lateral mass plate 2520 as well as secure the lateral mass plate 2520 to the engagement head 2606. The engagement seat 2612 receives the rear edge 2504 of the lateral mass plate 2520.

In one embodiment, the engagement head 2606 preferably includes the engaging rod 2616, as shown in FIG. 35B. The engaging rod 2616 is dimensioned to fit within the third bore 2502 in the lateral mass plate 2520. The engaging rod 2616 is coupled the switch 2608 on the handle 2602, whereby actuation of the switch 2608 causes the engaging rod 2616 to retract. Upon being retracted, the engaging rod 2616 disengages the third bore 2502 and allows the implant 2500 to be disengaged from the engagement head 2606. It is preferred that the tool 2600 includes a spring or other urging means to urge the engaging rod 2616 to the extended position, as shown in FIG. 35B. In another embodiment, the engaging rod 2616 is freely moveable between the extended and retracted positions without a biasing force applied thereto.

It should be noted that the engaging rod 2616 is shown as being a circular cylinder in FIGS. 35A and 35B. However, it is contemplated that the engaging rod 2616 can have any other shape which conforms to the shape of the third bore 2502 in the lateral mass plate 2520. In another embodiment, the engagement head 2606 does not include an engaging rod 2616 but some other mechanism to secure the implant 2500 to the tool 2600. In yet another embodiment, the slots 2612 in the side walls 2610 can be used to retain the implant 2500 in the head 2606 without the use of an engaging mechanism.

In preferred operation, to engage the implant 2500 with the tool 2600, the implant 2600 is oriented to be right side up such that the rear surface 2504 of the implant 2500 will conform and mate with the engagement seat 2614. The implant 2500 is aligned with the forked portion of the engagement head 2606, whereby the winged protrusions 2508 of the implant 2500 are inserted into the slot openings 2612. Upon registering the winged protrusions 2508 into the corresponding slots 2612, the lateral mass plate 2520 is guided into engagement by the slots 2612 until the rear edge 2504 mates with the engagement seat 2614. Preferably the engaging rod 2616 is then inserted into the third bore 2502, thereby securing the lateral mass plate 2520 to the engagement head 2606. In one embodiment, the user manually actuates the switch 2608 to retract the engaging rod 2616 to allow the lateral mass plate 2520 to be inserted completely in the receiving space. The switch 2608 is then manually released when the bore 2502 and engaging rod 2616 are aligned such that the engaging rod 2616 then extends and engages the third bore 2502. In another embodiment, contact between the superior surface of the lateral mass plate 2520 and the engaging rod 2616 causes the engaging rod 2616 to slightly retract while the plate 2520 is moved into the engagement seat 2614. Once the lateral mass plate 2520 is seated, the third bore 2502 preferably registers with the engaging rod 2616, whereby the urging force causes the engaging rod 2616 to automatically engage the third bore 2502.

During the surgical procedure, the natural or artificial facet joint spacer or inter-facet spacer 2510 is inserted into the distracted facet joint as described in detail above. Upon the natural or artificial facet joint spacer or inter-facet spacer 2510 being satisfactorily inserted in the facet joint, the user preferably actuates the switch 2608 to disengage the engaging rod 2616 from the third bore 2502. The surgeon then draws the tool 2600 away from the facet joint, whereby the lateral mass plate 2520 slides out of the received area and is guided by the slots 2612. The lateral mass plate 2520 is then anchored into the vertebral body as discussed above.

The foregoing description of the present invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many modifications and variations will be apparent to practitioners skilled in this art. The embodiments were chosen and described in order to explain the principles of the invention and its practical application, thereby enabling others skilled in the art to understand the invention for various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents. 

1-20. (canceled)
 21. An apparatus for sizing a facet joint in order to select a facet joint implant for implanting in the facet joint, the apparatus comprising: a handle; an interfacet sizer having a proximal end and a distal end with an upper concave surface and an opposing lower convex surface extending from the proximal end to the distal end, the upper convex surface and lower convex surface tapering toward each other and having a rounded distal end, the interfacet sizer connected with the handle at the proximal end; and a stop extending from the concave surface at the proximal end of the interfacet sizer, the stop adapted to prevent over-insertion of the interfacet sizer during sizing, wherein the interfacet sizer is angled obliquely from a longitudinal axis of the handle and sized to fit between an inferior facet of an upper vertebra of the facet joint and a superior facet of a lower vertebra of the facet joint.
 22. The apparatus of claim 21, wherein the distal end of the interfacet sizer has a distal tip, and wherein the stop extends a first distance from the longitudinal axis and the distal tip extends a second distance from the longitudinal axis, the second distance being greater than the first distance.
 23. The apparatus of claim 21, wherein the stop includes a substantially planar surface extending substantially perpendicular to the concave surface of the interfacet sizer, the substantially planar surface arranged to abut one of the upper and lower vertebras to prevent over insertion of the interfacet sizer during sizing.
 24. The apparatus of claim 23, wherein the handle includes a neck portion, the neck portion tapering towards the interfacet sizer and the interfacet sizer connected to the neck portion.
 25. The apparatus of claim 24, wherein the stop includes a tapered portion extending from the substantially planar surface towards the neck portion.
 26. The apparatus of claim 21, wherein the handle includes a substantially cylindrical portion having a radius extending from the longitudinal axis that defines a cylindrical boundary coincident with the cylindrical portion and extending along the longitudinal axis, and wherein the interfacet sizer is disposed within the boundary.
 27. An apparatus for sizing a facet joint in order to select a facet joint implant for implanting in the facet joint, the apparatus comprising: a cylindrical handle member extending along a longitudinal axis and having a proximal end and a distal end, the cylindrical handle member having a radius extending from the longitudinal axis that defines a cylindrical boundary coincident with the cylindrical handle member and extending along the longitudinal axis; a neck member extending from the distal end of the cylindrical handle member, the neck member having a proximal end, a middle portion, and a distal end with an exterior surface tapering from the proximal end towards the middle portion of the neck member, the proximal end, the middle portion, and the distal end each having a different width; an interfacet sizer having a proximal end extending from the distal end of the neck member and having an opposing rounded, tapered distal end to facilitate insertion of the interfacet sizer into the facet joint, the interfacet sizer having an upper concave surface and an opposing lower convex surface extending from the proximal end to the distal end of the interfacet sizer, the upper convex surface and lower convex surface tapering toward each other to form the rounded, tapered distal end, wherein the interfacet sizer is disposed within the boundary; and a stop disposed at the proximal end of the interfacet sizer, the stop including a first portion having a substantially planar surface extending substantially perpendicular to the concave surface of the interfacet sizer, the stop adapted to prevent over-insertion of the tool during sizing and including a second portion extending from the substantially planar surface and tapering towards the distal end of the neck portion.
 28. The apparatus of claim 27, wherein the distal end of the interfacet sizer has a distal tip, and wherein the stop extends a first distance from the longitudinal axis and the distal tip extends a second distance from the longitudinal axis, the second distance being greater than the first distance.
 29. The apparatus of claim 27, wherein the proximal end of the neck member has a first width, wherein the middle portion of the neck member has a second width, wherein the distal end of the neck member has a third width, wherein the first width is greater than the second width and the third width is greater than the second width.
 30. The apparatus of claim 27, wherein the handle has a handle thickness extending along the longitudinal axis and the interfacet sizer has a sizer thickness, the handle thickness being greater than the sizer thickness.
 31. The apparatus of claim 27, wherein the substantially planar surface extends along an entire width of the proximal end of the interfacet sizer.
 32. The apparatus of claim 27, wherein the stop has a pair of opposing substantially planar sidewall surfaces extending between the first portion and the second portion.
 33. The apparatus of claim 32, wherein the interfacet sizer has a pair of opposing substantially planar sidewall surfaces extending between the upper convex surface and the lower convex surface, and wherein the pair of opposing substantially planar sidewall surfaces of the stop are, respectively, substantially coplanar with the pair of opposing substantially planar sidewall surfaces of the interfacet sizer.
 34. The apparatus of claim 27, wherein the exterior surface of the neck member includes an upper surface and opposing lower surface extending from the proximal end to the distal end of the neck member, and wherein the convex surface of the interfacet spacer forms a continuous surface with the lower surface of the neck member.
 35. The apparatus of claim 34, wherein the lower surface of the proximal end of the neck member extends a third distance from the longitudinal axis and the lower surface of the middle portion extends a fourth distance from the longitudinal axis, the third distance being greater than the fourth distance.
 36. An apparatus for sizing a facet joint in order to select a facet joint implant for implanting in the facet joint, the apparatus comprising: a cylindrical handle member extending along a longitudinal axis and having a proximal end and a distal end, the cylindrical handle member having a radius extending from the longitudinal axis that defines a cylindrical boundary coincident with the cylindrical handle member and extending along the longitudinal axis; a tapering neck member extending from the distal end of the cylindrical handle member; an interfacet sizer having a proximal end extending from the tapering neck member and having an opposing rounded, tapered distal end to facilitate insertion of the interfacet sizer into the facet joint, the interfacet sizer having an upper concave surface and an opposing lower convex surface extending from the proximal end to the distal end of the interfacet sizer, a stop extending from the proximal end of the interfacet sizer, the stop includes a first portion having a substantially planar surface extending substantially perpendicular to the concave surface of the interfacet sizer adapted to prevent over-insertion of the tool during sizing and a second portion extending from the substantially planar surface and tapering towards the tapering neck portion.
 37. The apparatus of claim 36, wherein the tapering neck member has a proximal end, a middle portion, and a distal end with an exterior surface tapering from the proximal end towards the middle portion of the neck member, wherein the proximal end of the neck member has a first width, wherein the middle portion of the neck member has a second width, wherein the distal end of the neck member has a third width, wherein the first width is greater than the second width and the third width is greater than the second width.
 38. The apparatus of claim 37, wherein the exterior surface of the neck member includes an upper surface and opposing lower surface extending from the proximal end to the distal end of the neck member, and wherein the convex surface of the interfacet spacer forms a continuous surface with the lower surface of the neck member.
 39. The apparatus of claim 37, wherein the interfacet spacer has a fourth width and the fourth width is greater than the second width of the middle portion of the neck member.
 40. The apparatus of claim 36, wherein the stop has a pair of opposing substantially planar sidewall surfaces extending between the first portion and the second portion, wherein the interfacet sizer has a pair of opposing substantially planar sidewall surfaces extending between the upper convex surface and the lower convex surface, and wherein the pair of opposing substantially planar sidewall surfaces of the stop are, respectively, substantially coplanar with the pair of opposing substantially planar sidewall surfaces of the interfacet sizer. 